A passive matrix liquid crystal display ("LCD") is one popular type of display whose display elements respond to the values of the rms voltages applied across them. A passive matrix LCD includes overlapping row electrodes and column electrodes positioned on opposite sides of a film of liquid crystal material. The locations where the row and column electrodes overlap define the display elements. The portion of liquid crystal film associated with each display element is an electro-optic material that responds to a change in the value of an rms voltage applied across the display element to provide a corresponding change in the amount of light passing through it. The liquid crystal device most prevalently used in such displays is of an STN type.
The row electrodes receive addressing signals that select the rows at various times, and the column electrodes receive data signals that represent the information patterns to be displayed. A polarizer and analyzer, located on opposite sides of the STN cell, affect the intensity of light that propagates through the display element as the orientation of molecules in the liquid crystal material changes in response to electric fields applied to the liquid crystal material by way of the overlapping row and column electrodes that define that display element. A display system includes electrical circuits that provide a difference in electrical potential between overlapping row and column electrodes and thereby apply electric fields to the liquid crystal material.
Conventional low cost flat panel displays that display a high information content in matrixed form use STN LCDs almost exclusively. Such a display can be designed by following established guidelines, such as those summarized in T. Scheffer & J. Nehring, "Supertwisted Nematic (STN) LCDs" ("Scheffer & Nehring"), submitted to the 1993 SID International Symposium at Seattle, Washington. However, when following such guidelines, it is often difficult to design high contrast displays that are sufficiently bright and also have adequate contrast. In conventional low cost flat panel STN LCD displays the best compromise between contrast, panel uniformity, and cost of manufacturing has been achieved by using liquid crystal cells having a supertwist angle or layer twist angle of between 220.degree. and 240.degree.. Typically, both the polarizer and the analyzer are of a neutral density type.
However, because of the residual birefringence of the liquid crystal cell, in such a display the display elements appear colored. Film compensation or double cell compensation to achieve black and white displays is typically not practical where the objective is to achieve a low cost display.
Such a display typically falls into one of three categories of display coloration: a blue image on a yellow-green background, a dark purple-blue image on a pinkish background, and a dark blue image on a light gray-green background. Many people strongly dislike displays with blue on a yellow-green background. Displays showing dark purple-blue on a pinkish background are also not well-received.
The remaining conventional choice, dark blue on a light gray-green background, offers a color combination more appealing to most people. However, the intensity of the gray-green background of such LCDs is rather dark. As a result, reflective-type displays using that color combination are marginally adequate, and transflective-type displays using that color combination are just a bit too dark for effective use in dim or otherwise poor ambient light. (A transflective display may be viewed by a user in either light provided by a backlighting or illumination source contained in the display (the transmissive mode of use) or in ambient light (the reflective mode of use).) Attempts to enhance the viewing angle and to further diffuse the reflected light into a larger viewing cone make the darkness problem of the gray-green background even worse. Users of transflective displays implemented to provide that color combination tend to make frequent use of the backlighting source provided with such displays. This can create a severe battery life problem for portable devices equipped with such displays. In addition, displays showing dark blue on light gray-green tend to lose contrast at higher temperatures such as about 55.degree. C. (131.degree. F.); displays can reach such temperatures when in use.